An optical amplifier used in an optical transmission system has a control circuit to control pump light power based on input power and/or output power. In recent years, the control circuit of the optical amplifier may have been realized using a field programmable gate array (FPGA).
The FPGA includes an updatable program and/or a reconfigurable logic component, and thus it is possible to change a function of the FPGA by rewriting the program or reconfiguring the logic component. Therefore, by updating or upgrading the FPGA, it is possible to adjust an operation, a performance, a characteristic, and/or the like of the optical amplifier. Note that “updating (upgrading)” includes updating (upgrading) a program of the FPGA, reconfiguring a logic component of the FPGA, and/or updating (upgrading) a parameter of the FPGA.
The updating of the FPGA may be requested when an optical transmission system is in operation. In this case, the optical amplifier is supposed to have a function (in-service software upgrade function) of performing a process of updating the FPGA while continuing the operation of amplifying an optical signal.
However, the FPGA that controls the optical amplifier is not capable of controlling pump light power when the FPGA is being subjected to the updating operation. Therefore, when the updating of the FPGA is performed, for example, a control value for controlling the pump light power is stored in a memory. During a period in which the updating of the FPGA is being performed, the pump light power is controlled based on the control value stored in the memory. When the updating of the FPGA is ended, the control value stored in the memory is provided to the FPGA. Thus, the FPGA acquires the control value used immediately before the updating of the FPGA is performed, and the FPGA restarts controlling the pump light power.
As a related technique, an optical amplifier has been proposed which is capable of controlling optical amplification and updating an FPGA without conflict. As another related technique, an optical amplifier has been proposed which is for use at a relay node in an optical transmission system and which is configured to amplify an optical signal and perform an automatic level control (ALC) of output power. A method has also been proposed to suppress an error of a signal caused by an abrupt change in gain.
Descriptions of the above-described or other related techniques may be found, for example, in Japanese Laid-open Patent Publication No. 2007-220977, International Publication Pamphlet No. WO 2009/016695, Japanese Laid-open Patent Publication No. 2004-88376.